Previous work in this laboratory has shown that during bacterial growth under conditions in which glutamine synthetase (GS) activity decreases, antigenically cross-reacting material initially persists, suggesting that inactivation precedes degradation. Examination of the inactivating activity of cell-free extracts led to the development of a model inactivating system including only ascorbic acid, iron, and oxygen as necessary components. This and similar systems have been shown in this laboratory to inactivate a number of enzymes. Acid hydrolysis and amino acid analysis of the inactivated GS revealed the loss of one histidine residue per subunit as compared to the native control. It was undertaken to identify and isolate from the inactivated enzyme a peptide containing the altered histidine. A bacterial strain was constructed which overproduced GS allowed for the specific radiochemical labeling of the histidine residues so that the histidine containing peptides could be identified and the fate of the altered histidine traced. Trypsin and cyanogen bromide peptides of the native and inactivated GS were separated using reverse phase chromatography. A small, hydrophilic cyanogen bromide peptide from the inactivated enzyme has been tentatively identified which on amino acid analysis lacks one histidine compared to the corresponding peptides for the native enzyme.